The melt casting method is a method in which a melted film making composition, which is prepared by heating and melting polymers, is extruded from a die in a form of film, and solidified by cooling, and if needed, stretched, whereby a film is prepared. Since it is unnecessary to dry solvents, advantage results in which relatively compact facilities are employable.
On the other hand, it is common that the viscosity of melted polymers is higher than that of the polymer solution by a factor of 10-100. Consequently, since it is difficult to achieve leveling on the support, the resulting film carries problems in which clear streaking defects, called die-lines tends to occur. When the above die lines exist to a high degree, the following problems occur. When the resulting optical film is incorporated in a liquid crystal display device, light and shadow streaks, due to the die lines, are observed.
Specifically, melted materials of cellulose ester resins exhibit properties of no easy elongation due to high viscosity, whereby it has been difficult to employ the melt casting method. Specifically, when the draw ratio is high, thickness is not uniform in the film conveying direction (hereinafter, the film conveying direction or the conveying direction is occasionally described as the longitudinal direction), and problems occur in which breakage tends to occur in processes such as tenter stretching, whereby a decrease in die lines has been demanded. “Draw ratio”, as described herein, is a value which is obtained by dividing the lip clearance of a die by the average thickness of the film solidified on the cooling roller.
As a method to minimize the above defect, a method has been considered in which film is brought into closer contact with a cooling roller via a rigid metallic touch roller. However, it is anyhow impossible to eliminate unevenness of the touch pressure by employing this method. As a result, problems occur in which optical unevenness such as retardation increases.
In order to dissolve problems due to the use of the above metallic touch roller, a method is proposed in which an elastic touch roller is employed.
Patent Document 1 discloses that an optical film exhibiting optical unevenness is prepared by pressing a cooling roller against an elastic roller controlled at a specified film temperature during extrusion of amorphous thermoplastic resins. As the above elastic roller, a rubber roller covered with a metal sleeve is preferred. Further specifically employed is a silicone rubber roller (at a wall thickness of 5 mm) covered with a 200 μm thick metal sleeve.
Further, Patent Document 2 discloses that an optical film, which exhibits excellent thickness accuracy, is prepared in such a manner that during extrusion of amorphous thermoplastic resins, a touch roller, carrying a surface capable of being elastically modified, is brought into close contact with a cooling roller under the conditions that the contact width of the touch roller during no rotation and the cooling roller is specified. As the touch roller, employed, in the specific example, is a silicone rubber (at a wall thickness of 5 mm) roller of which surface is covered with a 200 μm metal tube.
In any of the examples described in the above patent documents, norbornene based resins are employed as polymers, and the effects are verified.
The inventors of the present invention carried out film molding via the elastic touch roller described in the above patent documents, employing cellulose ester resins, and evaluated the resulting optical film as a polarizing plate protective film. It became clear that problems occurred in which long-period optical unevenness and the same after storage over an extended period were observed. Further, when the film production rate was increased, it was noted that the long-period optical unevenness tended to be more marked. In each of the above patent documents, no film production rate is described. For example, at a film production rate of 15 m/minute, the optical unevenness was pronounced and productivity was deteriorated, whereby practical problems occurred. “Long-period optical unevenness”, as described herein, refers to the light and shadow unevenness which periodically appears over a length of approximately 10 cm, when a polarizing plate protective film is adhered onto a polarizing plate, followed by incorporation into a liquid crystal display device and the resulting device is employed at black display.
Further, Patent Document 3 discloses that it is possible to prepare an optical film exhibiting optical evenness by setting the temperature, just prior to close contact of an amorphous thermoplastic resin film with a cooling roller at Tg+30° C. or higher and by setting the draw ratio at 10 or less when the film thickness is 70-100 μm, at 15 or less when the same is 50-70 μm, and further at 20 or less when the same is at most 50 μm. Above Patent Document 3 describes that film is brought into close contact with a cooling roller via pressing or sucking, and describes various means. Specifically employed is an air chamber during molding of norbornene based resins and polysulfone resins.
On the other hand, when the method described in Patent Document 3 was employed, problems occurred in which die lines were pronounced, and when images were displayed via a liquid crystal display device, periodic light and shadow unevenness due to the die lines was observed.
As described above, at present, the polarizing plate protective film prepared via the melt casting film production method employing cellulose ester resins exhibits various problems.
Film composed of cellulose ester resins as a major raw material exhibits features such as optical evenness and minimal optical defects, in addition, excels in adhesion properties with a polarizer, and also exhibit features of appropriate moisture permeability when adhered onto a polarizing plate stretched via a wet system. Consequently, demanded is a manufacturing method of the polarizing plate protective film composed of cellulose ester resins as a major raw material via a melt casting film production method in which the various above problems are solved.
On the other hand, Patent Document 4 proposes application of 3-aryl-2-benzofranone as a stabilizer for organic materials which are easily subjected to oxidative, thermal, or photoinductive decomposition. As a specific example, Patent Document 5 discloses that in order to prepare a norbornene based polymer resin composition which excels in heat resistance, accurate molding properties, and stain resistant effects during molding at high temperature, compounds having a lactone structure are incorporated. Further, Patent Document 6 discloses that compounds having a lactone structure in the molecule or an acrylate group, and a phenolic hydroxyl group in the molecule are employed as a polymer carbon radical scavenger, whereby molded products are obtained which result in almost no formation of spot-like and streak-like external defects, coloration and color modification, and are appropriate for optical applications. However, as described above, when cellulose ester resins are employed, it is difficult to solve the problems due to the use of norbornene resins as a polymer.    Patent Document 1: Japanese Patent Publication Open to Public Inspection (hereinafter referred to as JP-A) No. 2005-172940    Patent Document 2: JP-A No. 2005-280217    Patent Document 3: JP-A No. 2003-131006    Patent Document 4: JP-A No. 7-233160    Patent Document 5: JP-A No. 2000-143946    Patent Document 6: JP-A No. 2002-121399